In elementary terms, a photoconductor is a piece of semiconductor material to which two electrical contacts have been affixed. When light is absorbed by the semiconductor, carriers are generated and the conductivity of the semiconductor between the electrical contacts changes. As is well known, a large number of semiconductor materials exhibit photoconductivity and they have found commercial use as, for example, light meters.
As only a single electron-hole pair is produced by each absorbed photon, it is not immediately apparent that photoconductors can exhibit current gain. However, a detailed analysis shows that when the lifetime of the photogenerator carriers exceeds the transit time of the carriers in the semiconductor, current gain can be obtained. In fact, extremely large current gains, e.g., 10.sub.6, can be obtained. The gain, as well as the associated gain-bandwidth product, can be controlled, within materials limitations, by appropriate choice of semiconductor characteristics such as carrier mobilities and lifetimes.
While gain is a desirable characteristic of phtodetectors, not all types of photodetectors necessarily exhibit gain. For example, the wellknown p-n junction, which is commonly used in photodetectors, does not exhibit current gain.